1. Field of the Invention
The present invention relates to a device and a method for channel simulation in a mobile communications system.
2. Discussion of the Background
At technical tests of mobile radio equipment, channel simulators (channel modulators) are used to emulate a realistic mobile radio channel under controlled forms. By means of a channel simulator, radio network functions such as handover, power control, and frequency hopping can be tested before they are used on a larger scale in the network.
The radio network function frequency hopping can be used in mobile communications systems in order to improve transmission performance by diversity gains for two differents disturbance causes. One quality of frequency hopping is interference diversity which may result in improvements in loaded networks. The other quality is generally called frequency diversity, and emanates from that the fading of the radio channel is not constant all over the system bandwidth. Frequency hopping is specified in GSM as an optional network function, i.e. the operator himself/herself decides if and when it shall be used. All mobile terminals, however, shall be able to cope with frequency hopping.
In the GSM specification there are channel models for type tests and evaluation, so called profiles, defined.
These profiles are designed in COST 207, and consist of a number, 6 or 12, discrete taps (so called “tapped delay line”-model) which fade independent of WSSUS (Wide Sense Stationary Uncorrelated Scattering). With tap here is referred to a signal path with defined fixed delay and average attenuation. The taps together constitute a model for the impulse response of the channels.
Channel simulators designed according to this type of channel model emulate the time dispersion of the channels very well for a system of medium data rate, type GSM. For real mobile radio channels, however, the strongest signals in the impulse response, which is modelled here by discrete taps, will vary. The degree of variation depends on environment and speed. The fixed taps according to COST 207 constitute a static model, which results in that the characteristics with regard to the frequency correlation is not modelled in a correct way. The frequency correlation in its turn decides which frequency diversity that is attained.
With that it is not possible to, in a realistic way, test frequency hopping with channel simulators which use such a fixed “tapped delay line”-structure.
The aim of the present invention consequently is to solve this problem and produce a channel simulator which in a realistic way can test frequency hopping.
To find out whether the prior art has solved this problem, a preliminary investigation was carried out, at which the following documents were found:
WO,A1,93/20626 describes a fading simulator where the input signal is led into FIR filter blocks connected in series, where the filter coefficients vary in time. See above all FIG. 3.
U.S. Pat. No. 4,105,958 relates to a channel simulator which makes use of two or more cascade connected delay lines with a plurality of taps to simulate channels with large dispersion of the time delays.
These documents, however, are impaired by the same statical model as the above described COST 207-model.